How to Repot a Young Japanese Black Pine

I have been growing Japanese black pine bonsai from seed for the past 17 years. During this time, I’ve done a lot of repotting. This past January, it dawned on me that I had to repot almost 50 young pines. Very few needed repotting last year, but this year all but a handful needed it. It seemed, at first, like more than I’d have time to complete. But instead contemplating the scope of the project, I simply picked up the first tree and got started.

Actually, I drove to Chinatown to pick up all of the colanders I could find. I’ve grown pines in both colanders and terra cotta pots. There are plusses and minuses to each approach. Although the terra cotta pots retain moisture better than the colanders, repotting with colanders is fast. This year I opted for speed.

About half way through I noticed the repotting was going quickly. I’d pick up a tree, cut back unnecessary growth, apply a wire or two and then repot. Here’s a breakdown of the process in two parts with special attention to the tools that make the work possible.

Colander

Colanders offer superb drainage. This is great because they allow me to water and feed the tree heavily. The downside is that the tree won’t develop quickly if I fail to water and feed heavily. As my goal is to thicken the trunks of my pines fast, the colanders will work well.

They’re also simple to wire.

Tie-down wires

Two wires will hold the tree in place. Number 2 aluminium is the typical choice for the job, but from experience I know that 1.5 mm aluminum will work well for me. Once the wire is in place, I add the drainage layer.

Drainage layer

One of the themes of this repotting season has been the attempt to use up all of the odds and ends of bonsai soil I’ve collected over the years. It felt great to go through so many buckets of bonsai soil – not because I like paying for it, but because I freed up a lot of space in my “workshop” (read: glorified patio). Above the lava drainage layer I added a small mound of medium sized bonsai soil – akadama with some pumice and lava.

Mound of bonsai soil – the pot is ready for the tree

I set the pot aside and turned to the tree. The pine seven is years old this spring from seedling cutting. I’m planning to make it into a small bonsai.

Japanese black pine – 7 years old

Some of the benefits of growing seedling cuttings is that they produce great roots and plenty of branches just perfect for small bonsai. To encourage the trunk to thicken, I’ve let the apex grow. The tree is over three feet tall.

Japanese black pine with escape branch

The steps I follow when repotting have been learned through years of workshops with Boon Manakitivipart and repeated lectures at Bay Island Bonsai meetings. For a live action version of the process, I highly recommend Boon’s videos on the topic.

The first step is removing the wires that hold the tree in the pot. This is made possible by the wire cutter.

Wire cutter for bonsai

I’ve used a number of wire cutters over the years. Any make or model that’s easy to work with will suffice. When removing the wires, I’m careful to cut very close to point where the wires emerge to ensure a nib doesn’t catch the edge of the pot and make it difficult to remove the tree.

Removing the tie-down wires

Tie-down wires removed

Once the wires are out of the way, I ease the tree out of the pot. Fortunately this didn’t take much work and the rootball came out intact.

The rootball out of the pot

The soil is moist – not too wet or too dry. Repotting when the soil is dry is hard on the tree. Repotting when the soil is too wet increases the odds that roots will tear and makes it hard to comb out the roots effectively. I begin the rootwork by combing out the roots at the bottom of the rootball. First, I turn the tree on its side so the bottom of the rootball is perpendicular to the turntable.

Upon turning this tree on its side, I noticed small white tips emerging from the rootball. These roots indicate that the tree is just waking up from winter – the perfect time to repot. Repotting when there are lots of new roots can slow the tree down. Repotting before the tree is active delays healing cut roots. This isn’t a problem where the winter is mild, but it can weaken trees exposed to hard freezes.

New roots emerging from the rootball – a perfect time to repot

The course root rake is one of my favorite repotting tools. It is the best way to carve through course roots circling in course drainage and makes quick work of the job. I use smaller rakes when the soil and roots are softer. This pine rootball is very firm and requires strong tools.

Root rake

It’s important to work at right angles when raking the bottom of the rootball. If I hold the tree or rake at any other angle, I risk removing more roots than I intend to remove, and I create a crater that’s hard to fill with soil when I place the tree in the pot.

Holding the root rake at the proper angle

Raking out the roots with a rasping or scraping motion

Once I’ve carved through about an inch of soil, I trim the loose roots with root scissors. This is one of the few indispensable bonsai tools.

Root scissors

Trimming the roots

When I trim the roots, I’m careful to leave a little bit of each root protruding from the rootbase. Cutting roots flush with the rootbase will make it harder for the tree to send out roots into the new soil.

Roots raked and trimmed – ready for the next step

Beginning the rootwork with the bottom of the rootball has one huge advantage over any other approach. Creating a flat surface allows me to set the tree on the workspace without damaging the rootball. Beginning with any of the sides makes it harder to balance the tree and increases the odds that I’ll destabilize the rootball.

Ready to work on the top and sides of the rootball

I usually work around the trunk of the tree with bent-nose tweezers, another indispensable tool. The fine points let me work carefully around the trunk without damaging the nebari.

Bent-nose tweezers

I hold the tweezers at a low angle to the rootball so the ends of the tweezers don’t catch and tear stubborn roots.

Combing out the roots

When roots prove too stubborn for tweezers, I use a root hook. These come in a number of sizes. The fairly large root hook below is perfect for this job.

Root hook

Using the root hook requires some care. Because the tool is so much stronger than tweezers and because the tip of the hook is curved more than the tweezers, it’s easy to catch and tear stubborn roots. Working at just the right angle and only scraping through a thin layer of roots with each pass protects delicate roots.

Combing the roots with a root hook

I’ll cover the remaining repotting steps – improving the nebari and tying the tree into the pot – next week. I will, however, mention a final repotting “tool.” I wear gloves when I repot as the soil is hard on my skin.

Glove

One of my favorite repotting activities is often neglected. After removing the tree from the pot and working with the small roots (see part one of How to repot a young Japanese black pine) it’s time to improve the nebari. “Nebari” refers to the area where the roots emerge from the trunk – it’s the one part of the roots we can see from the outside. Good nebari conveys strength. A weak or unsightly rootbase can detract from the overall look of the tree.

We can improve the nebari by removing roots that don’t fit in for one reason or another, or by arranging roots in a more attractive fashion. For a young pine with lots of roots, I rarely hesitate to remove troublesome roots because I know there are better roots underneath. If the roots to be removed are bigger than what root scissors can handle, I use a concave cutter.

Concave cutter

I have two of these – one for branches and one for roots. I’ve heard that this practice prevents the spread of fungus, though I don’t know how important this is. I like using separate tools for roots and branches because it’s easier to keep the branch-cutting tool sharp – the root tools must cut through soil particles – and I tend to keep my repotting tools separate from my other tools.

Some roots are easy targets. The small root below crosses directly above a root that grows directly away from the trunk.

Crossing root

Crossing root removed

Other roots are less likely candidates for removal. The largest root on this tree sits just above the soil line and snakes along the surface before diving below. I don’t like this for several reasons. Large roots that dive into the soil are little better than trunks that dive straight into the soil. In good nebari, we can see the transition from trunk to surface roots that appear to clutch onto the surface of the soil. All along the rootbase of this pine, very small roots emerge from the trunk with the exception of this large root. Removing it will reveal more small roots. As these small roots that emerge from the trunk grow and thicken, they’ll help increase the size of the trunk over time and create taper. For all of these reasons, I think this pine will be better off without the big root.

Large root

Large root removed

In general, the goal at this stage of development is to thicken the trunk and nothing else. Removing large roots slows this process to some degree. For this reason, I’ve removed some, but not all of the problematic roots – I can remove the others later when the tree is ready for refinement.

Rootwork complete – ready for the pot

Placing the tree in the pot is fairly straight forward when trees are in development. I don’t need to worry about finding the front of the tree – my main concern is that I plant it at the appropriate level. I begin by setting the tree in the pot on top of the mound of soil.

Rootball sitting atop the mound of soil

Next, I nestle the rootball into the soil until the nebari reaches the appropriate depth.

Tree nestled into place

Next come the tie-down wires. I connect the wires together by hand and then tighten them with pliers.

Bonsai pliers – general purpose pliers work well too

Tightening the wires that hold bonsai in place requires surprising care. Simply twisting the pliers in place makes it easy to break wires and difficult to ensure that wire is secure. After pinching the wires with the pliers, I pull the wires tight, and then slowly release the pressure as I twist the wires into place. Or as Boon says, “Pull, then twist!”

Grabbing the tie-down wires with pliers

Pull, then twist

Once the tree is secured in the pot, I add soil and work it into place with chopsticks. When the roots are snug in new soil, I fill the pot to the appropriate level and the repotting is complete.

Scoop for bonsai soil

Pouring in bonsai soil

Chopsticks

Repotting complete

As soon as the repotting is complete, I water the tree until the water drains clear. Removing the bonsai soil dust encourages good drainage.

Water wand for bonsai

Watering the tree

With the right tools for the job, repotting a single young pine can be a quick process.

My name is Jonas Dupuich and I am a Northern California bonsai enthusiast. I have been studying and writing about bonsai under the guidance of Boon Manakitivipart for the past sixteen years, and I am a founding member of Bay Island Bonsai.

This article originaly appeared in “Florida Bonsai” the magazine of the Bonsai Societies of Florida. VOL XXXV NUMBER 2 ISSUE 146 Reprinted with permission of the publisher of “Florida Bonsai” magazine. Attack of the killer mites [...]